1. Field of the Invention
The present invention relates to an illumination apparatus having a reflecting mirror for reflecting light from a light source toward an object to be illuminated.
2. Description of Related Art
In a known illumination apparatus, a reflecting mirror is provided behind a light source, so that not only light directly emitted from the light source, but also light reflected by the reflecting mirror are made incident upon an object to be illuminated, thus resulting in a more efficient utilization of light. The reflecting mirrors that are used, for example, are elliptic mirrors, parabolic mirrors, or optical aspherical mirrors.
For instance, in the case of an elliptic mirror, when a light source is located at a first focal point thereof, light emitted from the light source and reflected by the mirror surface is converged onto a second focal point. The light utilization efficiency increases as the extension of the mirror surface from the apex thereof on the first focal point side increases. However, by increasing this extension, the diameter of the mirror is also increased, resulting in a large mirror. Such a large mirror requires a bright and expensive projection lens (i.e., a projection lens having a small F-number) to fully converge and project the light reflected by the elliptic mirror onto an object (e.g., a chart), because the angle between the light flux reflected by the elliptical mirror and the optical axis of the mirror increases.
In the case of a parabolic mirror, light emitted from a light source located at a focal point and reflected by the mirror surface is emitted therefrom to be parallel with an optical axis thereof. FIG. 16 shows the relationship between a focal length "f" and an emission light flux diameter ".phi." in a parabolic mirror R, provided that an acceptance angle S of light from a light source L is constant. When an object (e.g., a chart) is small, it is desirable to decrease both the focal length "f" and the emission light flux diameter ".phi.". However, if the focal length "f" is decreased, there is a possibility that a reflecting surface area of the parabolic mirror is made too small to effectively reflect light, depending on the size of the light source L. Furthermore, since the distance between the light source L and the reflecting surface is decreased, light energy per unit reflecting surface area, becomes large, resulting in an accelerated deterioration of the coating layer of the reflecting surface and decreased heat radiation within the parabolic mirror.
On the other hand, if the focal length "f" is increased, the light flux diameter ".phi." is larger than necessary to cover the object. This results in an increased loss of light. To prevent this, it is possible to use a relay lens to make the light flux diameter ".phi." smaller to the minimum diameter required to cover be identical to the size of the object. However, in this case, the angular magnification becomes larger, so that the angle of intersecting light rays becomes larger. Consequently, it is necessary to use a bright projection lens (i.e., a projection lens having a small F-number), which is very expensive, in order to increase the light gathering efficiency.